Photosensor array integrated circuits are organized as an array of photosensor cells, each photosensor cell having a photodiode or phototransistor together with transistors arranged to precharge a capacitance of the photosensor cell prior to an exposure, the capacitance may be a junction capacitance of a photodiode. The photodiode or phototransistor is typically arranged to discharge the capacitance proportionally to light received on the photodiode or phototransistor. The photosensor cell also has transistors arranged to read charge from the capacitance. The array may have one or more defective photosensor cells, or photosensor cells that do not respond to light in quite the same way as most other photosensor cells of the array. During production, photosensor arrays are tested and those arrays having excessive or objectionably defective or nonuniform photosensor cells are discarded, as large arrays are more likely to have defective cells, the larger the array the greater the cost.
Photosensor arrays typically are formed with a “Bayer-pattern” of color filters over the photodiodes or phototransistors of the photosensor array; typically in macrocells of four cells. Each macrocell can only provide one full three-color signal, as such, inherent spatial resolution of color is significantly less than the stated pixel count of the array because color detection requires multiple photodiode or phototransistor cells. In a typical Bayer-pattern macrocell, three cells are each covered by a different color filter, such as a red, a green, and a blue filter, the fourth photosensor of the cell may be uncovered or may be covered with a color filter duplicating one of the other three cells.
At low light levels, such as at night, photosensor arrays with large photodiodes and/or a long integration time are desirable to capture sufficient photons to give adequate intensity resolution; however high spatial resolution images typically require large numbers of photodiodes. Image sensors having large numbers of large photodiodes are much more expensive than sensors with small numbers of large photodiodes, or with large numbers of small photodiodes; they also are more likely to have defects than sensors with smaller surface area.
Typically a portion of each photosensor array's active surface is occupied by the precharge and selection transistors necessary to prepare each photodiode for exposure, and to read exposed photodiodes. Other space is often consumed with interconnect and guardrings intended to reduce crosstalk between adjacent photodiodes. Shapes, even bright lines, well-focused on the image sensor array may not be captured in an image if they land entirely between photodiodes.
There are situations—such as tripod-based landscape and still-life photography—where photographers wish for very high resolution images of scenes that undergo little, if any, movement during an exposure.